The present invention relates generally to aluminum hydride, or xe2x80x9calane,xe2x80x9d and more particularly relates to a novel method for preparing aluminum hydride polymorphs such as xcex1-AlH3 and xcex1xe2x80x2-AlH3. The invention additionally relates to a stabilized form of xcex1-AlH3, to energetic compositions, particularly propellant compositions, containing, as a fuel, stabilized xcex1-AlH3 and/or xcex1xe2x80x2-AlH3 prepared using the method of the invention, and to methods of using the alane polymorphs prepared herein as chemical reducing agents, as polymerization catalysts, and as a source of hydrogen gas such as in batteries and fuel cells.
Aluminum hydride, also referred to as xe2x80x9calane,xe2x80x9d is usually prepared as a solution by the reaction of lithium aluminum hydride with aluminum trichloride. A. E. Finholt et al. (1947) J. Am. Chem. Soc. 69:1199. The alane-containing solution, however, is not stable, as an alane.ether complex precipitates from solution shortly after preparation. In addition, attempts to isolate the nonsolvated form of alane from the ether solution result in the decomposition of the complex to aluminum and hydrogen. M. J. Rice Jr. et al. (1956) Contract ONR-494(04) ASTIA No. 106967, U.S. Office of Naval Research.
In a method for preparing non-solvated alane, alane.etherate may be desolvated in the presence of a small amount of lithium aluminum hydride. See, for example, A. N. Tskhai et al. (1992) Rus. J. Inorg. Chem. 37:877, and U.S. Pat. No. 3,801,657 to Scruggs. Non-solvated alane exhibits six crystalline phases, with each having different physical properties. The phase designated as xcex1xe2x80x2-alane is essentially non-solvated and appears under a polarizing microscope as small multiple needles growing from single points to form fuzzy balls. The xcex3 phase appears as bundles of fused needles. The xcex3 phase is produced in conjunction with the xcex3 phase, and both xcex3- and xcex2-alane are metastable nonsolvated phases that convert to the more stable xcex1-alane upon heating. The xcex1-alane is the most stable, and is characterized by hexagonal or cubic shaped crystals that are typically 50-100 xcexcm in size. The other two forms, designated xcex4 and xcex5-alane, are apparently formed when a trace of water is present during crystallization, and the xcex6-alane is prepared by crystallizing from di-n-propyl ether. The xcex1xe2x80x2, xcex4, xcex5 and xcex6 polymorphs do not convert to the xcex1-alane and are less thermally stable than the xcex1-form. For a discussion of the various polymorphs, reference may be had to F. M. Brower at al. (1976) J. Am. Chem. Soc. 98:2450.
Alane consists of about 10% hydrogen by weight, thereby providing a higher density of hydrogen than liquid hydrogen. Because of the high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, alane can be used as a fuel for solid propellants or as an explosive.
Solvated alane can be synthesized by the reaction of LiAlH4 with aluminum chloride, resulting in the alane.etherate complex (equation 1). 
In an alternative synthesis, LiAlH4 is reacted with sulfuric acid to give the alane.etherate complex (equation 2). 
The AlH3-ether complex is then treated with a mixture of LiAlH4 and LiBH4, and heated (equation 3). 
The combination of LiBH4/LiAlH4 enables use of a lower processing temperature, and xcex1-alane is the final product after heating at 65xc2x0 C. under vacuum. In an alternative synthesis, Bulychev reports that xcex1-alane can be prepared at pressures greater than 2.6 GPa and at temperatures in the range of 220-250xc2x0 C. B. M. Bulychev et al. (1998) Russ. J. Inorg. Chem. 43:829. Under those conditions, apparently only the xcex1-alane form is observed.
In addition, alane can be directly synthesized by metathesis of aluminum alkyls followed by removal of the alkylaluminum byproduct in vacuum (equation 4). 
Still another method of preparing nonsolvated alane is by bombarding an ultrapure aluminum target with hydrogen ions. However, alane thus produced has poor crystallinity.
One of the obstacles to large scale production of xcex1-alane is the handling of the diethyl ether solution of the alane.ether complex. At concentrations of about 0.5 M or higher and temperatures above 0xc2x0 C., the alane.ether phase prematurely precipitates out of solution. In addition, xcex1-alane can be contaminated with other phases of alane, and is not stable over time as the complex decomposes to hydrogen and aluminum.
Thus, although alane is potentially promising as a high energy density fuel, because of its high hydrogen density and the highly exothermic combustion of aluminum and hydrogen, the lack of a suitable method for synthesizing alane in a stabilized form has severely limited its applicability.
Accordingly, it is a primary object of the invention to address the above-mentioned need in the art and provide a method for synthesizing xcex1-alane in a stabilized form.
It is another object of the invention to provide stabilized xcex1-alane as a novel composition of matter, prepared using the aforementioned method.
It is an additional object of the invention to provide a method for synthesizing xcex1xe2x80x2-alane.
It is a further object of the invention to provide energetic compositions containing stabilized xcex1-alane or xcex1xe2x80x2-alane, prepared using the methods described herein.
It is still a further object of the invention to provide such energetic compositions in the form of a propellant composition.
It is also an object of the invention to provide methods for using stabilized xcex1-alane or xcex1xe2x80x2-alane, prepared using the methods described herein, as an energy dense fuel, as a chemical reducing agent, as a polymerization catalyst, and as a source of hydrogen gas such as in batteries and fuel cells.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one embodiment, then, the invention relates to a method for preparing stabilized xcex1-AlH3 wherein: (a) an alkali metal hydride is initially reacted with AlCl3 in diethyl ether to form an initial AlH3 product and an alkali metal chloride; (b) the reaction mixture is filtered to remove the alkali metal chloride; (c) an excess of toluene is added to the filtrate of step (b), resulting in a diethyl ether-toluene solution; (d) the diethyl ether-toluene solution is heated and distilled to reduce the amount of diethyl ether in solution, until a precipitate is formed; (e) the precipitate is isolated; and (f) the isolated precipitate is added to an acidic solution effective to dissolve and thus remove materials other than xcex1-AlH3. In a preferred embodiment, the acidic solution contains an xcex1-AlH3 stabilizing agent, e.g., a compound that coordinates to the Al3+ ion, an electron donor, or an electron acceptor.
In another embodiment, the invention provides a method for synthesizing xcex1xe2x80x2-AlH3 wherein (a) an alkali metal hydride is initially reacted with AlCl3 in diethyl ether to form an initial AlH3 product and an alkali metal chloride; (b) the reaction mixture is filtered to remove the alkali metal chloride; (c) an additional alkali metal hydride and an excess of toluene are added to the filtrate of step (b), providing a diethyl ether solution containing xcex1xe2x80x2-AlH3 and optionally other AlH3 polymorphs; and (d) removing the xcex1xe2x80x2-AlH3 is from the solution.
In a further embodiment of the invention, a propellant composition is provided containing fuel, a binder material, and an oxidizer, wherein the fuel is a stabilized xcex1-AlH3 product or an xcex1xe2x80x2-AlH3 product prepared using the aforementioned techniques. The alane polymorphs of the invention are compatible with a wide range of binder materials, oxidizers, secondary fuels, and other propellant components, and provide for a propellant that is chemically and physically stable over an extended period of time.
The invention also provides methods for using the stabilized xcex1-AlH3 product and the xcex1xe2x80x2-AlH3 product in other contexts. For example, an alane polymorph as synthesized herein may be used as a chemical reducing agent, in any context wherein a hydride donor is appropriate to bring about reduction, e.g., in reducing unsaturated carbonxe2x80x94carbon bonds such as present in alkenes and alkynes, in reducing carbonyl-containing moieties such as ketones, aldehydes, carboxylic acids, and acid chlorides, in converting halides to hydrido moieties, and the like. The present alane polymorphs may also be used as polymerization catalysts, typically in catalyzing addition polymerization reactions, e.g., the polymerization of olefins, and as a hydrogen source in fuel cells and batteries.